This invention relates to a jitter detection circuit, and more particularly to a circuit for detecting a jitter of an image signal reproduced from a recording media.
When an image signal is reproduced from a recording media such as a magnetic tape, magnetic disc and optical disc, the reproduced signal includes a variation component of time base (jitter) due to a variation of transit speed of a recording media and other cause. Such a jitter deteriorates a reproduced picture quality. A reproduced image signal without the jitter can be generated by reading it after writing the reproduced image signal on a time base corrector (hereinafter called as `TBC`) utilizing a memory. In this case, it is necessary to apply a jitter detection signal to the TBC as a writing clock by detecting a jitter of the reproduced image signal.
FIG. 9 shows a block diagram of an example of the conventional circuit for detecting a jitter. The conventional jitter detection circuit is used as an automatic frequency control (hereinafter called as AFC) circuit of a processing circuit for a reproduced carrier color chrominance signal in a home VTR using a helical scan method.
In helical a home VTR of helical scan method, there is widely used a VTR of low band conversion color recording and reproducing method, wherein a color image signal is separated into a luminance signal and a carrier chrominance signal, the luminance signal is converted as frequency-modulated luminance signal, the carrier chrominance signal is modulated in a band lower than that of the frequency-modulated luminance signal, and a signal formed by a frequency division multiplex of the two signals is recorded on a magnetic tape and reproduced by a rotating head.
Accordingly the VTR employing this method comprises a processing circuit of a reproduced carrier chrominance signal for converting a reproduced low band conversion carrier chrominance signal in an original frequency band, and an AFC circuit for generating a frequency conversion signal having a jitter as same as that of the reproduced low band conversion carrier chrominance signal in order to reduce a jitter included in a reproduced low band conversion carrier chrominance signal in converting the frequency of the reproduced low band conversion carrier chrominance signal.
With reference to FIG. 9, a reproduced luminance signal is fed to a horizontal synchronizing signal separation circuit 2 through an input terminal and a horizontal synchronizing signal separated by the separation circuit 2 is supplied to one input terminal of a phase comparator 3. The phase comparator 3 compares a phase of the reproduced horizontal synchronizing signal from the horizontal synchronizing signal separation circuit 2 with that of a horizontal scanning frequency signal from a frequency divider 6, with the result that the phase comparator 3 generates an error voltage corresponding to the phase difference therebetween.
A loop filter 4 integrates the error voltage and applies its result to a voltage control oscillator 5 (hereinafter called as `VCO`) as a control voltage.
The VCO 5, for example, has a self oscillating frequency of 320 f.sub.H (f.sub.H is a horizontal scanning frequency and 15.73426 KHz in a NTSC method). The frequency divider 6 divides an output oscillating frequency of the VCO 5 by 320 and transmits its result, a horizontal scanning frequency f.sub.H, to other input terminal of the phase comparator 3.
Therefore the VCO 5 generates an output oscillating signal of 360 f.sub.H or close to 360 f.sub.H phase-synchronized with a reproduced luminance signal put into the input terminal 1. Because the reproduced luminance signal has the same jitter as the reproduced low band conversion carrier chrominance signal, the output oscillating signal comes from an output terminal 7 and becomes a frequency conversion signal for converting the frequency band of the reproduced low band conversion carrier chrominance signal into an original frequency band after being converted into a given frequency.
Considering a dubbing in a carrier chrominance signal recorder, the circuit as illustrated in FIG. 9 is installed for producing a carrier of a low band conversion chrominance signal phase-synchronized with a horizontal synchronizing signal extracted from an input image signal by the horizontal synchronizing signal separation circuit 2. In the circuit of FIG. 9, the low band conversion carrier chrominance signal is inserted at a given period (1/4 in a NTSC and 1/8 in a PAL) within a horizontal scanning cycle.
In a VTR of low band conversion color recording and reproducing method, a low band conversion carrier chrominance signal is demodulated into an original frequency band by a reproduced carrier chrominance signal process circuit having a AFC circuit, and meanwhile a reproduced color image signal is generated by multiplexing a reproduced carrier chrominance signal having a reduced jitter and a reproduced luminance signal demodulated by a reproduced luminance signal process circuit.
The reproduced color image signal is transmitted to a color monitor, and is displayed as the reproduced color image signal which is practical and free of a jitter visually by utilizing a feature of a AFC and a APC (Automatic Phase Control) of the color monitor.
However a jitter of the reproduced luminance signal of the reproduced color image signal is not reduced at moreover not all and all jitter of the reproduced carrier chrominance signal is eliminated completely. So a TBC is used so as to completely eliminate jitter included in a reproduced color image signal in order to secure a picture image of high quality or a complete frequency interleaving between the reproduced luminance signal and the reproduced carrier chrominance signal.
But if an output oscillating signal of the VCO 5 which is a jitter detection signal detected by a conventional jitter detection circuit is used as a writing clock of the TBC, a the conventional jitter detection circuit is a sampling system of a horizontal scanning period and a feed back control circuit, and a visibility of the loop filter 4 requires a comparatively great value of 15 H (H is a horizontal scanning period). With the result that the follow-up of the jitter detection circuit is inferior.
In the conventional jitter detection circuit, a tape/head impact error jitter of high frequency occurred caused by a vibration of a magnetic tape just after contacting a rotating head or a skew occurred by an elasticity of a magnetic tape are not followed up sufficiently and a jitter detection signal can not be used as the writing clock.